for future reference as its still fresh in my head, chasing a lamba sensor fault yesterday, we found a couple of interesting things.
Engine running, with the crypton ACT, the reading is in millivolts ( 0.001v), the true voltage the lambda sensor outputs. with the sensor disconnected the reading was fixed at 403mv (0.403v). connecting a sensor that was not in the exhaust and sitting freely on the plastic oil filler cap, the sensor tip started to warm up as would be expected (careful, very very hot, burny burny fingers!), but the volt reading remained 403mv. simply placing the body of the sensor onto something bare metal with a ground path the volt reading instantaniously changed to 200mv, a reading consistant with high oxygen content (it is in the air of course), suggesting the grounding of the sensor body is an impotant factor. not sure if this is just a cheapo after market sensor thing, but i would always recommend a little extra money is spent in buying a Bosch part as i've rarely had issues with them other than damge or old age (bit like me )